1. Field of the Invention
The present invention relates generally to electronic measuring instruments, and in particular to electronic instruments having transducers that produce electrical signals in response to physical quantities and electronic circuitry that processes those signals.
2. The Prior Art
Electronic instruments using transducers that produce electrical signals indicative of the magnitudes of physical quantities are known to the art. Examples of such instruments include electronic scales and gas flow controllers. Typically, a transducer employed by such an instrument is connected as an arm of a bridge circuit, and when a physical quantity to be measured (such as weight, gas flow, or the like) is sensed by the transducer, the bridge becomes unbalanced in an amount related to the magnitude of the quantity being measured. The degree of unbalance of the bridge is manifested as an electrical signal, and this bridge signal is amplified by a bridge amplifier to yield an output signal that in turn can be used for such purposes as activation of a display or control of a process that depends on the magnitude of the measured quantity.
Ideally, the bridge signal would be directly proportional to the magnitude of the physical quantity being measured, because such proportionality is necessary in order to produce a usable readout from the signal. However, because presently available transducers are rarely capable of producing a signal that is proportional to the magnitude of the measured quantity, such direct proportionality can only be achieved by applying a correction factor to the bridge signal. The process of deriving and applying such a correction factor is referred to herein as the process of "equalizing" the bridge signal.
A useful method of equalizing a bridge signal, known to the art, consists of applying a fixed correction signal to an amplifier stage. Such a correction signal alters the gain of the amplifier stage, causing the signal at the output of the amplifier to be proportional to the magnitude of the quantity being measured. Unfortunately, any one correction signal only yields an exactly correct output for one single value of input, and therefore an infinite number of correction signals, one for each possible input signal value, would theoretically be required to make the output signal proportional to the magnitude of the measured quantity over the entire range of the instrument. Fortunately, however, in practice as few as four correction signals, each operative over a limited range of input signal values, are sufficient to cause the output signal to be sufficiently close to proportional to the magnitude of the physical quantity being measured that the instrument will give acceptably accurate results over its entire range.
One method of generating such correction signals is to use a plurality of inverting operational amplifiers. Each such amplifier receives as its input the bridge signal. The output of each such amplifier is configured to alter the gain of an amplifier stage in the bridge amplifier, and each operational amplifier is biased so that it will not produce an output until its input exceeds the magnitude of the bias. By carefully choosing the bias levels to be applied to the various operational amplifiers, the circuit can be so configured that each operational amplifier will correct the gain over a different portion of the range of the instrument, resulting in an overall output response curve that is very close to perfectly proportional to the measured quantity.
If each transducer of a given kind could be counted on to have nearly the same response characteristic as every other transducer of the same kind, then a single set of appropriate bias values could be derived and used to bias the operational amplifiers in each instrument employing such a transducer. However, there is often a wide variance in response characteristics from one transducer to another, and bias values must therefore be individually derived for each instrument using such a transducer. The need to derive such bias values anew for each instrument tends to make the circuitry more complex than it would otherwise have to be in that adjustments for all the bias sources must be provided, and it tends to make the alignment process more complex in that each bias source must be individually aligned according to the individual response characteristic of the transducer used in that instrument, a costly and time-consuming process.
It will be apparent from the foregoing that there is a need for an electronic measuring instrument employing a plurality of operational amplifiers to equalize the response characteristic of the transducer that does not require individual alignment of the bias sources to coincide with each different transducer. The present invention satisfies this need.